The present invention relates to a diffraction grating for dispersing the light emitted from a light source every wavelength in a spectrophotometer.
As shown in the specification of U.S. Pat. No. 3,045,532, as a diffraction grating for dispersing the light every wavelength, a diffraction grating in which a grating constant of a groove is constant is used.
FIG. 1 is a diagram showing a cross section which is vertical to a ruled groove of a conventional Echellette type diffraction grating. A number of triangle grooves 2 are formed on a surface 3 of a diffraction grating 1 at regular intervals of d (mm). The interval d of the grooves 2 is also called a grating constant.
In FIG. 1, when an incident light beam 5 enters at an angle .theta. for a grating normal 4 which is vertical to the surface 3 of the diffraction grating 1 and a diffraction light beam 6 is diffracted at an angle .phi., the relations among the angles .theta. and .phi. and a wavelength .lambda. (nm) of diffraction light 6 are expressed by the following well-known equation. EQU N.multidot..lambda.=d.multidot.(sin.theta.-sin.phi.) (1)
where, .lambda.: wavelength of diffraction light beam, PA1 d: grating constant, PA1 .theta.: angle between the incident light beam and the grating normal, PA1 .phi.: angle between the diffraction light beam and the grating normal, PA1 N: degree (integer) of diffraction light. PA1 .DELTA..lambda.: width of spectrum which is emitted from the slit having the width .DELTA.s PA1 f: focal distance of camera mirror PA1 N: degree of diffraction grating
An Echellette type diffraction grating which has been put into practical use is manufactured in a manner such that the shape of groove 2 is formed into a triangle, thereby enabling about 90% of the energy of the diffraction light beam to be concentrated when the degree N=1 and N=2. Therefore, it is possible to consider such that in the equation (1), the degree N has the values of EQU N=1 and 2 (2)
FIG. 2 is a block diagram showing a constitution of a spectrophotometer using the Echellette type diffraction grating shown in FIG. 1. The white light beam emitted from a light source 11 passes through a lens 12 and is transmitted through a specimen chamber 13 and enters an incident slit 14 of the spectrophotometer. Thereafter, the light transmitted through the slit 14 is converted into a parallel light beam 16 by a collimating lens mirror 15 and enters a diffraction grating 17. The parallel light beam diffracted at a constant angle by the surface of the diffraction grating 17 enters a camera mirror 19 and is focused onto an outlet slit 20. A wavelength of light beam focused on the outlet slit 20 becomes a wavelength of monochromatic light determined by the equation (1). The monochromatic light enters a photomultiplier 21 and is converted into an electric signal. Thereafter, the electric signal is arithmetically processed in a signal processor 22 and displayed by a display device 23.
In such a spectrophotometer, in order to adjust the wavelength of monochromatic light which is focused to the outlet slit 20 into a desired wavelength, in the central portion of the surface where the light enters the diffraction grating 17, a straight line which is parallel with the groove is used as a rotary axis 24 and the diffraction grating 1 is rotated by only a constant angle by a wavelength drive section 18.
Now, assuming that the collimating lens mirror 15 and camera mirror 19 are fixed, the sum of the incident angle .theta. and diffraction angle .phi. of the diffraction grating 17 becomes constant. Namely, it can be expressed by the following equation. EQU .theta.+.phi.=2K.sub.0 ( 3)
When the equation (1) is rewritten using the equation (3), we have EQU N.multidot..lambda.=2d.multidot.sin (.theta.-K.sub.0).multidot.cosK.sub.0 ( 4)
In this case, since EQU sin(.theta.-K.sub.0).multidot.cosK.sub.0 .ltoreq.1 (5),
there is a limitation in selection of the wavelength .lambda.. Namely, ##EQU1## If the primary light (N=1) having the largest intensity of diffraction light is used, we have EQU .lambda..ltoreq.2d (7)
Namely, in the conventional apparatus shown in FIG. 2, there is an important drawback such that when a grating constant of diffraction grating is set to d, a wavelength range which the spectrometer can select is limited to a value below 2d (mm).
On the other hand, a dispersion power (spectrum width to a unit slit width) of conventional spectrophotometer shown in FIG. 2 is expressed by the following equation. ##EQU2## where, .DELTA.s: slit width (it is assumed that a width of incident slit is equal to a width of emitting slit)
Assuming that N=1 in the equations (6) and (8), the following points will be understood.
(I) The grating constant d of diffraction grating needs to be set to a large value in order to widen a wavelength range which the spectrophotometer can select.
(II) The grating constant of diffraction grating needs to be set to a small value in order to raise a dispersion power of spectrophotometer.
Namely, the spectrophotometer using a conventional diffraction grating has a characteristic such that the wavelength range and dispersion power are contradictory. Thus, there is a problem such that a dispersion power must be reduced to widen a wavelength range and, on the contrary, a wavelength range must be narrowed to improve a dispersion power.